When a flat bed scanner is used to scan bindings of thick documents such as a book and a magazine as shown in FIG. 1, a binding portion 41 of a book document 40 is scanned such that the binding portion 41 cannot be laid flat on a contact glass 2 of an image reading apparatus 1. In FIG. 1, the binding portion 41 of the book document 40 is laid on the contact glass 2 substantially in parallel to a main scanning direction. When the binding portion 41 of the book document 40 rises above the contact glass 2, the binding portion 41 is away from a focal point of the scanner, so that both ends of a binding boundary line of the book document 40 are distorted and a scanned image shrinks in the main scanning direction and sub-scanning directions as shown in FIG. 2.
As shown in FIG. 1, the image reading apparatus 1, configured as part of a digital copier 16, includes a pivotable platen 17 and a sensor 18. The platen 17 contacts a document to the contact glass 2. The sensor 18 detects an opening and closing of the platen 17. A document scale 11 determines a position for placing a document and adjusts the contact glass 2.
As shown in FIG. 2, various technologies are proposed for correcting a distortion by expanding a distorted, scanned image in main and sub-scanning directions.
Japanese Laid-Open Patent Publication No. 2002-358514 describes a method for checking curvatures of straight lines such as a book outline (i.e., a page outline), a ruled line, and a character line in a document, measuring a magnification for correcting the curvatures to original straight lines, and expanding the curvatures in the main scanning direction.
In a correction in the sub-scanning direction, when a location in the main scanning direction of an optical axis passing through a center of a lens of a scanner and a distance from the lens to a contact glass are determined, a distance from the contact glass 2 to a height of the book document 40 is determined so that a three dimensional position is identified. Using information about the three dimensional position allows for expanding in the sub-scanning direction.
In addition, Japanese Laid-Open Patent Publication No. 2002-247643 describes a method for use in a ratio of a rectangular width and height circumscribing a character in a scanned image when a location in the main scanning direction of an optical axis passing through a center of a lens of a scanner and a distance from the lens to a contact glass are not determined.
Presently, the above-mentioned techniques, however, possess their own distinct disadvantages. Users who require a distortion correction cannot perform a process of a correction, causing a longer processing time and a lower correction image quality. In addition, after the process of correction is completed, a corrected image often has a different size from an original document.